1. Field of the Invention
The present invention relates to a dynamic visual-vestibular acuity testing protocol that can utilize a compact rapid response laser light source capable of forming fast cycling (e.g. faster than 75 mille-seconds) shapes and images in which the motion of the display may be easily and precisely matched to the motion of the patient and the patient's eyes.
2. Brief Description of the Prior Art
Humans use three basic systems, namely visual, vestibular, and proprioceptive, to obtain a sense of balance in daily life. The three systems interact to maintain posture and impart a conscious sense of orientation in space. There are measurable reflexes associated with these stimulus modalities. Reflexes generally serve to maintain stability in posture (e.g. by extending muscle groups in the direction of an anticipated fall), or in maintaining stability of the visual field, thereby suppressing dizziness. A defect in one of these systems, or incongruous inputs amongst the systems can be compensated by a patient through reliance on the other two systems. However, such a defect decreases the patient's overall ability to adjust to incongruous stimuli between the other two fields. Also, a defect can result in a serious subjective feeling of disequilibrium in the affected patient until compensation for the deficit occurs.
Regarding the visual system, visual inputs aid in the maintenance of an upright posture and aid in orientation. Conscious and unconscious correction of posture is possible through processing of visual inputs. The adjustment of posture and sensation of movement in response to visual stimuli can be seen by observing individuals' responses to opto-kinetic stimuli (repeated movement of large objects in the subject's visual field). Such stimuli (e.g. a train moving on the adjacent platform) impart a sense of acceleration to the individual and lead to reflexive postural adjustments (e.g. leaning in the direction of the moving train) to maintain balance. Visual reflex arcs also aid in maintaining the stability of the visual field. The saccade system focuses a visual target of interest onto the fovea through a fast movement of the eyes. The smooth pursuit system allows fixation of gaze onto a moving object with a frequency of less than 1.2 Hz. The optokinetic reflex is a result of multiple objects moving through a patient's visual field (with the moving objects occupying about 80% of the patient's visual field). The optokinetic reflex imparts a sense of motion to the patient. It presents as a jerk nystagmus with the slow component in the direction of the moving objects and the fast component back to the midline.
Regarding the proprioceptive system, proprioceptive inputs will aid in static and dynamic postural control primarily through two reflex arcs. The first is the myototic reflex (deep tendon reflex), in which stretch on a muscle causes contraction of the muscle. The myototic reflex serves to maintain stability across a joint. The second proprioceptive reflex arc that aids in posture control is the functional stretch response, which utilizes multiple somatosensory inputs to provide for coordinated limb and trunk movements across joints, for instance to maintain the center of gravity over the support base in an individual who is bumped from behind. This reflex pathway has a higher latency than the myototic reflex, although both are mediated through spinal pathways. Both of these reflex arcs have lower latencies than visual-postural reflexes and vestibular-postural reflexes.
Regarding the vestibular system, the vestibular system consists of two groups of specialized sensory receptors: the semicircular canals and the otolithic organs. The semicircular canals detect angular acceleration of the head. The three semicircular canals consist of a membranous semicircle with a widened area, the ampulla, at one end. The ampulla contains the crista ampullaris: specialized ciliated cells (several small cilia and one large eccentric kinocilium on each cell) jutting into the lumen of the ampulla. The cilia are embedded in a gelatinous structure called the cupula. The membranous semicircular canal contains endolymph (extracellular fluid with a high potassium concentration) that is the same specific gravity as the cupula. When angular acceleration of the head occurs in the plane of the semicircular canal, the endolymph's momentum causes it to stay relatively stationary, displacing the cupula slightly. The cupula then displaces the cilia, causing a decrease in the firing rate of the associated vestibular nerve if the cilia bend away from their kinocilium and an increase in vestibular nerve firing if the cilia bend toward their kinocilium. There are 6 total semicircular canals (three in each inner ear): paired lateral, superior and posterior. The lateral semicircular canals have a plane that is elevated 30 degrees from the coronal. The posterior and superior semicircular canals are in planes that are approximately 90 degrees from each other and both are laterally askew from the sagital plane. The superior (a.k.a. anterior) canal on one side is on the same plane as the posterior canal on the other side, and so detects angular acceleration in the same plane. The ampulla and kinocilia in the crista ampullaris of the lateral semicircular canal are arranged in such a way that ampullopetal flow of endolymph causes increased firing of the vestibular nerve and ampullofugal flow decreases the firing of the vestibular nerve. The arrangement is opposite to this in the posterior and superior canal, with ampulopetal flow leading to inhibition of the associated vestibular nerve branch and ampulofugal flow leading to excitation. The end result is the same though: when head is turned towards the side of the semicircular canal (in its plane), that side's vestibular nerve is excited and the opposite paired side's nerve is inhibited. The range of response in excitation of a nerve is greater than the range of response in inhibition. Therefore, both vestibular nerves are generally required to sense acceleration without any detectable deficit.
The otolithic organs consist of a utricle, which is oriented the axial plane, and a saccule, which is oriented in the sagittal plane. These structures contain ciliated cells underneath a gelatinous layer and are bathed in endolymph. They also contain otoconia, which are calcium carbonate crystals of a higher specific gravity than endolymph. The otoconia are displaced in response to changes in head position with relation to the vertical. The otolithic organs also respond to linear acceleration. The ciliated cells can inhibit or excite the vestibular nerve, depending on the direction of their bend in relation to the kinocilium (away=inhibition, towards=excitation).
The vestibular system can affect posture via vestibulospinal pathways. These pathways, in conjunction with visual-postural and proprioceptive-postural pathways, serve to maintain the patient's center of gravity over the base of support. For instance a quick head tilt to the right causes extension of right sided leg extenders to counteract a change in the perceived center of gravity. A perceived forward motion causes a sway forward to maintain the support base. The vestibulo-ocular reflex is a system that maintains the stability of the visual field in response to acceleration of the head in a particular direction. The pathway is from vestibule to vestibular nuclei to the ocular motor nuclei, with modulation from cerebellar centers. The reflex results in movement of the eye so that the fovea can focus the same image during movement of the head. Thus the eye rotates (including tortional rotation) in an exactly opposing fashion to the head. When the eye's rotational limit is exceeded, a saccade brings the eye back to the midline. For example, rotation of the head (nose) to the left would result in excitation of the left branch of the vestibular nerve that innervates the left semicircular canal and in inhibition of the right semicircular canal branch. This combination of excitation and inhibition is passed through the reflex arc and is translated into excitation of the ocular muscles to rotate the eye to the right in an exactly opposing fashion to the head rotation until no longer possible, at which point a saccade brings the eye back to the midline. The process then repeats itself until the angular acceleration ceases.
The vestibular system is a very important system in the conscious sensation of acceleration. Peripheral or central damage to the vestibular system would lead to a severe sense of imbalance until compensation occurs. Also, they would result in measurable alterations of the vestibulo-spinal and vestibulo-ocular reflexes until compensation occurs. Compensation in peripheral vestibular injury is via adjustment of the gain of vestibular reflexes in the cerebellum and modification of signal delivery to supratentorial centers.
There is a growing need, both in the US and worldwide, to improve the health care provided for people who suffer from hearing, balance, and vestibular function disorders. Ninety million Americans go to their health care providers because of vertigo, dizziness or balance problems each year. In the general population, 20% is estimated to be affected by a vestibular disorder with the prevalence increasing with age according to a University of Iowa Health Care study. Hearing and balance problems are the third ranking chronic condition in the older population, yet 75% of those who have a hearing and/or balance disorder are not addressing this issue properly. Dizziness or loss of balance, which the National Institutes of Health estimates will occur in 70% of the nation's population at some time in their lives, is the second most common complaint heard in physicians' offices.
The elderly represent a substantial and growing segment of the population suffering from balance disorders. According to the Department of Otolaryngology at the University of Kansas, approximately 12.5 million Americans over the age of 65 have dizziness or balance problems that significantly interfere with their lives. The increasing elderly population, the rising cost of care, and the need for an expansion of treatment protocols, underscore a growing interest in the vestibular field. More importantly, they are leading to a substantial increase in the need for new technologies and delivery mechanisms that improve vestibular assessment and care.
In the ocular and the vestibular testing field, a wide variety of lighting devices have been developed for providing visual stimuli to the patient for testing. For example Neuro Kinetics, Inc. has provided, since the early 1990's, a laser light device for use with a vestibular testing rotary chair. This laser device was called the Pursuit Tracker™ device. The slow response time of the device (cycle time of about 50 Hz, acceleration of about 1000 degrees/second2 and velocity of about 1200 degrees/second) limited the applications of this device. The prior art device had a position accuracy and repeatability of about 1-2 degrees. Consequently the applications of this device were for testing in which a single visual point or marker was utilized (the single point of the prior Pursuit Tracker™ device could be moved to provide a moving marker to the patient).
There is a need in the industry to provide a compact neuro-otologic testing device producing rapid response or fast cycling (e.g. faster than 75 mille-seconds) shapes and images and in which the motion of the display may be easily matched to the motion of the patient. There is a further need in the industry to develop testing to exploit this technology to provide new tools for neuro-otologic testing and treatment.
Turning to a specific application of the testing platform of the present invention, aircraft pilots, ground vehicle drivers and marine craft skippers all must operate effectively in a physically and visually complex and dynamic environment. Just as critically, the myriad equipment operators, passengers, and support personnel on these different platforms must also be able to perform their function whenever called on. Errors and failures due to motion sickness, whether arising from a natural predisposition or solely from the environment, must be reduced to save lives, equipment and money. Research by the United States Navy has identified that 13.5% of aircrew experienced motion sickness. In a study of Royal Air Force student pilots 15% of the students experienced motion sickness that was severe enough for the flight to be abandoned. The pilot student motion sickness can lead to costly delays in the flying training, and problems associated with student's low self-esteem. For non-pilot aircrew, the probability of becoming motion sick is much higher than for pilots if the motion/visual stimulus is severe enough. Paratroopers experience higher (between 10 to 75%) rate of motion sickness.
While motion sickness in student pilots is not uncommon, it occurs in relatively controlled environments with significant safety systems in place. Once training is completed, the risks of motion sickness can remain a problem. The affected aircrew member can become a liability to the crew and a potential flight safety hazard, depending on the nature and circumstances prevailing at the time. It has been observed that pilots exposed to motion and visual stimuli may eventually become adapted to the stimuli and their motion sickness substantially reduced and at times even disappears. This indicates that treatment and effective screening may be possible.
Three classes of treatments for individuals who suffer from motion sickness exist—behavioral treatments, pharmacological treatment, and desensitization programs.
Behavior Treatments—Biofeedback training, relaxation and psychological counseling can achieve a success rate of up to 40% by reducing the conflict between visual and motion stimuli. For example, motion-affected individuals can view a stable visual reference (such as the horizon), as this will minimize the visual-vestibular conflict. While this is an effective in some cases, it is not generally applicable to military operating environments where constant attention to nearby displays, operation in confined spaces, or operation in the dark may be required.
Pharmacological Treatments—A variety of anti-motion sickness medications have been used to assist pilots in training and other personnel. They are not suited for chronic use, and due to the anti-cholinergic side effect, they are not acceptable for use by solo pilots. They are, however, acceptable for short-term use in student pilots in order to allow them to become adapted to their new three-dimensional operating environment while flying with an instructor. It has been determined that use of medications does not suppress the eventual acquisition of the motion-adapted state, and may actually facilitate adaptation by allowing continued motion exposure and reducing anxiety during the desensitization period.
Desensitization Programs—Many air forces around the world use desensitization programs for the treatment of chronic and intractable motion sickness. Desensitization exploits the fact that repeated exposure to a motion stimulus eventually results in a degree of protective adaptation being conferred on the individual. Desensitization programs generally involve a gradual, incremental exposure on a repetitive basis to a provocative motion stimulus. The motion most often utilized in such programs is the cross-coupled or Coriolis phenomenon. Such a program generally takes a number of weeks to complete, largely determined by how quickly the individual adapts to increasing speeds of rotation. In the Royal Air Force, the ground-based Coriolis stimulation is followed by a flying phase, which consists of a structured program of increasingly provocative and aggressive maneuvering in a jet training aircraft. The success rate of the RAF program is a return to flying of 85%.
The United States military also has a desensitization program, which incorporates the use of biofeedback to enhance the efficacy of the training, with a reported success rate of 79%. The individual who has undergone desensitization must return to the dynamic flight environment as soon after the program is completed for lasting benefit. Any significant delays to the resumption of flying training may lead to loss of the protective adaptation to motion achieved through the desensitization program.
The desensitization approach is the most attractive, since it can be adapted to mimic military requirements, unlike the behavior treatments, and does not have the negative side effects or time restriction of drug treatments. However, objectively measuring subject performance, both prior to and during treatment, is still difficult. Without such a measurement, effectively implementing and evaluating screening and desensitization efforts is impossible.
As early as the 1980s, it has been demonstrated that patients with vestibulotoxicity had difficulty reading standardized acuity charts during active head movements performed at the bedside. There have been some commercial versions of a dynamic visual acuity (DVA) test, all using active patient head movements. These prior DVA tests, as a screening test for vestibular function, have been studied for the aviation community. High frequency voluntary head rotation (i.e. the patient vigorously shakes his head back and forth) was selected as a vestibular stimulus to isolate (VOR) from visual influence. It was determined that DVA tests at frequencies ˜2 Hz can be used to evaluate vestibular function. Studies have determined that DVA tests during imposed head motion are a quantitative and clinically feasible measure of oscillopsia reflecting functionally significant abnormalities of the VOR. Further studies have shown that DVA measured during forced head-on-body or whole-body transient motion can correlate closely with VOR performance only if optotypes are presented during directionally and temporally unpredictable, high-acceleration head motion. From these past results, the inventor of the present invention believes that performing DVA testing while the subject is experiencing a simulated operating environment will provide a measure of performance, and performance improvements due to a desensitization program.
It is one object of the present invention to provide an efficient, effective integrated system, consisting of motion simulator and DVA testing subsystems, that may be used for determining the susceptibility of personnel to motion sickness, and also for desensitization treatments. An object of the present invention is that such a platform will provide a screening function that will save time and money by identifying people with severe or intractable problems early. The rehabilitation function of such a platform can safely help the personnel and verify readiness to return to operations. It is a further object of the invention that such a combined system be fully integrated, rugged, portable, easy to use by nonexperts, and fully validated. It should be capable of simulating real operating conditions, including helmets and other gear the personnel will be using.